Inner classes and nested classes in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested class in Kotlin?

A nested class is a class defined inside another class without the <code>inner</code> keyword. It does not hold a reference to the outer class.

beginner

What is an inner class in Kotlin?

An inner class is a class defined inside another class with the <code>inner</code> keyword. It holds a reference to an instance of the outer class and can access its members.

beginner

How do you declare an inner class in Kotlin?

Use the <code>inner</code> keyword before the class name inside the outer class. For example:<br><pre>class Outer {
  inner class Inner {
    fun greet() = "Hello from Inner"
  }
}</pre>

intermediate

Can a nested class access members of the outer class directly?

No, a nested class cannot access members of the outer class directly because it does not hold a reference to the outer class instance.

intermediate

How can an inner class access the outer class members?

An inner class can access outer class members directly because it holds a reference to the outer class instance. You can also use <code>this@Outer</code> to refer to the outer class explicitly.

Which keyword makes a class inside another class hold a reference to the outer class in Kotlin?

A static

B nested

C outer

D inner

The inner keyword makes the nested class hold a reference to the outer class instance.

Can a nested class in Kotlin access the outer class's properties directly?

A No, never

B Only if marked inner

C Yes, always

D Only if outer class is static

A nested class without the inner keyword cannot access outer class properties directly.

How do you create an instance of an inner class in Kotlin?

A val inner = Outer.Inner()

B val inner = Outer().Inner()

C val inner = Inner()

D val inner = new Outer.Inner()

You must create an instance of the outer class first, then call Inner() on it.

What is the default nature of a nested class in Kotlin if no keyword is used?

A Static nested class

B Companion object

C Anonymous class

D Inner class

By default, nested classes in Kotlin are static and do not hold a reference to the outer class.

Which expression refers explicitly to the outer class instance inside an inner class?

A this

B super

C this@Outer

D outer.this

Use this@Outer to refer to the outer class instance inside an inner class.

Explain the difference between inner classes and nested classes in Kotlin.

Think about how each class relates to the outer class and their access to its members.

Describe how to create and use an inner class instance in Kotlin.

Remember the inner class needs the outer class instance to exist.